Heretofore electrooptical phase modulators have been used in which the application of an electrical field causes an induced index of refraction change in the waveguide. This index change is applied in such a manner that one part of the propagating optical beam, i.e., one polarization, suffers a phase retardation relative to the remaining part of the optical beam, i.e., the other polarization. This phase retardation manifests itself in a rotation of the polarization linearly polarized light generally becoming elliptically polarized. The prior art method of producing a modulator with this effect is to place an active crystal between two polarizers. The application of an electric field rotates the incoming polarization and light is either passed or blocked by the analyzer polarizer. Bulk electrical optical modulators based on the above description have been set forth in "Modulators for Optical Communication" by F. C. Chen, in Proc. IEEE, Vol. 58, No. 10, pages 1440-1457, October, 1970.
Electrooptical modulators are believed to play an important role in the emerging integrated optics technology. In addition to the above, the following systems are set forth; "Thin Film LiNbO.sub.3 Electro-Optical Light Modulator" by I. P. Kaminaro et al, in Applied Physics Letters, Vol. 22, No. 10, pages 540-542, May 15, 1973; "Waveguide Electro-Optic Modulation in II and VI Compounds" by W. E. Martin, Journal of Applied Physics, 44, page 3703, 1973; and "Dielectric Thin FIlm Optical Branching Waveguide" by H. Yajima, Applied Physics Letters, Vol. 22, No. 12, pages 647-649, June 15, 1973.
Thin film integrated optical form presents problems that preclude a simple extension of bulk modulator techniques in fabricating a thin film electrooptic amplitude modulator. A major problem is that usually the phase retardation is applied between the TE and TM modes. Unless these modes are degenerate and coupled, it is not possible to mix them within the waveguide. Therefore, in order to mix these modes, one must allow the beam to propagate out of the waveguide. In such systems, one can use external polarizers or heterodyne detectors to study such phase modulation. However, the use of external polarizers or detectors defeats the purpose of integrated optics which combines the light within the system. Therefore it is desired to obtain a method of changing the phase modulation in a waveguide into an amplitude modulation while the beam is still within the optical waveguide.